1. Field of the Invention
The present invention relates to an overcurrent limitation circuit employed in a direct current stabilization electric power supply circuit.
2. Discussion of the Background
FIG. 1 illustrates a conventional overcurrent limitation circuit as a first example. Such a circuit includes; a stabilization electric power supply that controls an output transistor M1 to output a constant amount of Vout in accordance with a signal obtained by amplifying a difference between a voltage, obtained by dividing the output voltage Vout with resistors R1, R2, and R3, and a reference voltage Vref using a differential amplifier (AMP); a differential amplifier block, wherein an input to a transistor M6 is a voltage obtained by dividing the output voltage Vout, and an input to the other transistor M5 is a voltage obtained by converting a current, which flows through a monitor transistor M2 in proportion to that carried in an output transistor M1, into a voltage with a resistor R4, and a transistor M7 is included so as to form a source follower circuit and give an offset to the voltage; and a control transistor M8 whose operation is controlled by an output from a differential amplifier block and controls a control line of an output transistor M1 between an output of an operational amplifier and an electric power supply voltage Vdd.
An operation of the circuit of FIG. 1 is described with reference to an output characteristics illustrated in FIG. 2. A current flowing through the transistor M2 is small when no load in connected to a current output terminal to relative to when a prescribed load is connected thereto during a normal operation. An input voltage to the transistor M5 is sufficiently smaller than that to the transistor M6. An input to the control transistor M8 is a high voltage, and an output Vout is constant due to turning OFF of the control transistor M8.
As an output current Iout increases and the input voltage to the transistor M5 rises, the input voltage to the transistor M8 declines. When the transistor M8 turns ON, since the input voltage of the transistor M1 is withdrawn to the electric power supply (i.e., VDD) side, an output current (Iout) is limited and the output voltage Vout starts descending.
Since the input voltage to the transistor M6 also descends as the output voltage Vout descends, an output of a differential block turns ON the transistor M8, when a current giving an input voltage of the transistor M5 and flowing through the transistor M2 decreases up to a prescribed level. In addition, the output current having a proportional amount thereto also decreases.
When the output voltage Vout is a ground level, the input of the transistor M6 is also the ground level. Due to a threshold voltage Vetch of the offset transistor M7, the input to the transistor M5 does not become zero, and is a stable point while a current (i.e., short current) flows through the output transistor M1. Both the resistors R1 and R2 can be neglected if current limitation is set in appropriate.
In a case of the exemplary circuit of FIG. 1, a value of a limit current is necessarily determined not to excessively flow through a load when a value of a short current is determined. Further, as understood from FIG. 2, in a case of a regulator capable of varying an output voltage, the lower the output voltage Vout, the smaller the value of the limit current is. As a result, current supplying capability sometimes is not maintained as a problem.
FIG. 3 illustrates a second example of an overcurrent limitation circuit 2. The overcurrent limitation circuit 2 includes a pair of circuits including a limit circuit and short limitation circuit. Since the short limitation circuit in the right side is substantially the same to the circuit 1 of FIG. 1, its description is omitted.
The exemplary circuit 2 additionally includes a limit circuit, and obtains output characteristics as shown in FIG. 4 by alternating the above-described two circuits at a prescribed point in the drawing.
When the output voltage Vout remains high, the output from the differential amplifier block of the short limitation circuit is high as described earlier, and the transistor M8 is turned OFF. Similar to the transistor M2, a current flowing through the transistor M9 in proportion to that carried in the transistor M1 is flowed to a resistor R5 by current mirror circuit formed from transistors M10 and M11. When a flowing current is large, a gate voltage for the transistor M12 becomes low, and a gate voltage for the output transistor M1 rises. As a result, a current flowing through the output transistor M1 is limited.
When the output voltage Vout becomes low, a gain of the right side short limitation circuit becomes higher, and the current (Iout) is further limited, thereby a curvature approaching the short current value Is and having an offset (i.e., a current value is not zero) is drawn.
This exemplary circuit 2 of FIG. 3 can separately set either a limit current value or short current value. However, a circuit configuration becomes complex due to two circuits and a necessary area becomes large in this circuit. Further, since a current limitation value (i.e., the minimum voltage causing the limit current) determined by the two circuits is fixed, optimal protection characteristics are hardly obtained.